Coal, formed from decomposed and compressed vegetable matter, is typically found in substantially horizontal seams extending between sedimentary rock strata such as limestone, sandstone, or shale. Surface and underground mining are the primary techniques used to recover this coal.
Surface or strip mining involves removal of material (known as overburden) overlying a coal seam so as to expose the coal for recovery. In recent years, surface mining has gained prominence over underground mining in the United States. This is due to many factors including: (a) the increased material moving capacity of surface or strip mining equipment; (b) lower costs for surface mining than underground mining; (c) the better safety record of surface mining versus underground mining; (d) a higher coal recovery percentage for surface mining versus underground mining; and (e) many coal reserves favor extraction by surface mining due to geologic factors.
Surface mining does, however, have its limitations despite the advantages cited above. The primary limiting factor relates to the depth of the overburden. Once the coal seam reaches a certain depth below the surface, the amount of overburden that must be removed to reach the coal simply makes strip mining economically unfeasible.
Once this depth is reached, large quantities of coal may still remain in the ground and other mining methods must be utilized if economic recovery of this coal is to be achieved. Underground mining application in such an instance is, however, very limited. This may be due to poor roof support conditions, the thinness of the seam and/or the presence of insufficient quantities of coal to warrant the large capital investments characteristic of underground operations.
Due to these considerations, auger mining is often used to recover coal following a strip mining operation where the overburden becomes too costly to remove. A large auger is used to bore into the face of the seam and recover the coal from beneath the overburden. Advantageously, auger mining is very efficient providing more tons per man day than any other form of mining. Auger mining also may be activated quickly and requires a relatively low capital expenditure when compared to surface and underground mining.
Thus, auger mining may be used to supplement a strip mining operation and to recover smaller coal deposits. Auger mining really does not compete with underground mining that requires the large reserves, as well as the substantial capital and time to develop. Auger mining is the best method to use in relatively thin seams. Further, auger mining is safer than both surface and underground mining, while also providing a generally cleaner coal product.
Auger mining is, however, also not without its disadvantages. Despite the high rate of production, auger mining provides a relatively low total coal recovery. Coal recovery for the resource area being augered is usually less than about 35%. Some of the lost recovery is due to the pillars of coal that are left standing to support the overburden between adjacent auger holes. The majority of the recovery shortfall, however, is due to the limited penetration depths achievable with the present auger mining equipment.
The present augering equipment characteristically drills holes that sag downwardly gradually with increasing depth of penetration into the coal seam. Eventually, the auger tends to drill through the bottom of the coal seam into the underlying rock layer. Disadvantageously, this can lead not only to dirty coal (i.e. coal contaminated with other materials) and reduced coal recovery per drilling foot, but also to expensive cutting head damage and even cutting head loss. Thus, in actual practice, each auger hole is started at the top of the seam and drilled 30% under size (i.e. in relation to the height of the coal seam) to allow for the sagging caused by the weight of the cutting head and drill steel during auger operation.
Further, as penetration depths increase, a greater number of auger flights are required to convey the coal from the cutting head to the seam face for recovery. Each flight adds to the frictional resistance to the turning of the auger through contact with the walls of the bore hole. Additionally, the longer the string of auger flights, the greater the weight of coal being moved by the flights at any one time. As a result, it should be appreciated that auger power requirements increase rapidly with the depth of auger penetration.
Holes drilled with conventional augering equipment are usually only of a depth of 150 feet, with 200 feet being rarely obtainable. Of course, any increase in this figure is desirable as it would greatly improve the coal recovery rate from a resource area.